Aluminum oxide ceramics (also known as alumina ceramics), both monolithic materials and aluminum oxide based composites, as well as other aluminum based ceramics, are a widely used class of materials. Uses for these materials include furnace tubes, spark plug insulators, cutting tool inserts, and substrates for electronic components among others. In comparison to most other structural ceramics, the alumina ceramics possess outstanding corrosion resistance and dielectric strengths, but only modest mechanical strengths. As is the case with other structural ceramics, the alumina ceramics fail to reach their full potential with respect to mechanical strength because of strength-limiting surface flaws that are invariably introduced during fabrication. It is well established that the strengths of these materials are generally only one tenth to one hundredth of that which theoretically should be attainable. The reason for this behavior is that ceramics fail from the stress induced propagation of microscopically small cracks, not from an overstress condition. These detrimental flaws may originate either from defects such as pores, particle agglomerates or microcracks introduced into the material during the sintering process, or as a result of later operations such as slicing or grinding. Because the failure of a ceramic material is so dependent on the population and distribution of such flaws, the strength of the ceramic is statistical in nature, depending on the probability that a flaw severe enough to cause fracture at a given applied stress is present in the volume of material that is exposed to the peak stress. Thus it follows, that the observed strength is related to the volume or surface areas under stress and the number and severity of flaws in that volume area.
It has been experimentally determined that the most damaging flaws in a ceramic body are those at, or very near, the surface. Surface flaws are particularly detrimental at a surface which experiences tensile forces, since cracks tend to open and propagate under tensile forces. Thus, it can be seen that the strength of a ceramic material can be increased by altering the surface region to remove, or reduce the number or size of strength limiting flaws. This may be accomplished in some cases by polishing the surface to a mirror like finish, but this is an expensive, time consuming process that is not a practical solution for most commercial ceramic products.
The use of strengthened aluminum based ceramics would be beneficial in present applications because thinner cross-sections would be possible, thus saving on materials and allowing for overall smaller components. Greatly improving the strength of the aluminum based ceramics might also allow these materials to be used in applications which at present require stronger, costlier materials, such as silicon nitride or silicon carbide ceramics. Furthermore, while earlier work set forth in USSIR H1166 entitled "Process for strengthening silicon based ceramics" discloses a process for strengthening ceramics similar to the process of the present invention, it is limited to structural ceramics containing silicon, and does not teach or suggest a method that can be used to strengthen aluminum based ceramics that do not contain silicon as in the present invention. Moreover, the level of strengthening achieved in the method of the present invention is almost twice that achieved for silicon based ceramics. Accordingly, there is a need for an economical process to improve the strength of aluminum based ceramics.